The invention relates to an externally ignited four cycle internal combustion engine in which the fuel-air mixture is formed outside of the at least one cylinder and where each cylinder is provided with at least one dependent outlet valve. The opening to be closed by the outlet valve is disposed in the upper portion of a swirl or turbulence chamber which is located substantially within the cylinder head, and within which the mixture to be combusted is substantially contained when the piston is in its top dead center position. The lower edge of the turbulence chamber within the cylinder head or of the turbulence chamber region within the cylinder head is located at least substantially within a cylindric geometric surface defined by the track of the piston within the cylinder. A dependent inlet valve is disposed outside of the turbulence chamber and it preferably may be the only inlet valve of any particular cylinder.
In a known internal combustion engine of this type (U.S. Pat. No. 2,763,254) the turbulence chamber is very large and overlaps nearly one-half of the cross section of the interior volume of the cylinder and the outlet valve is located in the cylinder head, as are three spark plugs. The top face of the piston is provided with recesses and a median depression is arranged in such a way that, during the compression stroke, gas flows into the two outer end regions of the turbulence chamber and forms vortices which rotate in mutual symmetry and in the opposite sense. This results in high flow losses, a low thermodynamic degree of efficiency and it also requires the presence of three spark plugs and has further disadvantages.